Why is the collector- emitter voltage (Vce) not getting biased?

Question

This question is an extension for my previously asked question here: Why is the voltage across collector and emitter pushing itself to 0.7 volts?

A colleague of mine suggested that I take a look at the stability factor for the circuit in that question. I calculated it and found it to be 284, which is very high and bad, and I thought that this is what was driving the transistor into saturation. But then re-did my calculations by assuming a lower stability factor of 10. From this, I got new values of resistors as follows:

R1 = 4.7 K ohms

R2 = 1.2 K ohms

Rc = 2.2 K ohms

Re = 100 ohms

After making these changes, when I tested the DC conditions by applying a supply voltage of 5 volts, there were no changes in the results. The Vce voltage still remains at 0.7 volts, i.e almost in saturation, contrast to my expected value of 2.5 volts. Here are the various voltage drops I get:

Across R1 : 4.6 volts

Across R2: 0.4 volts

Across Rc: 4.2 volts

Across Re: 0.2 volts

I have even tried changing the transistor. The transistor I used is 2N3904C NPN. Please enlighten me and provide me with a solution on how to overcome this.

Answer 1

On this question, and on your previous question, you keep getting 0.7 volts Vce.

Perhaps you have swapped the base and collector leads. This would put the base emitter leads across your "emitter" resistor, resulting in 0.7 volts no matter how you bias the resistors R1 and R2.

2N3904C

EDIT : If you have connected transistor properly, you can place a jumper across your R2 (short it out, making it zero ohms), and Vce should rise dramatically. Do not leave the jumper installed after this test.

After satisfying that your circuit is wired properly, increase value of R1 (perhaps to 6.8K) and increase in small steps until you see Vce begin to rise in voltage.

I have added a schematic to represent what the OP has in his previous question, and with his current values.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Your main problem is that, following the answer to your previous question, you made the RC/RE ratio too high. Consider what is going on:

1) R1/R2 set the base voltage at ~1 volt.

2) This sets the emitter voltage at ~0.3 volts (give or take a bit).

3) So the emitter current should be ~3 mA.

4) The collector current should be about the same as the emitter current, so the collector resistor should have a voltage of (3ma x 2.2k) or about 6.6 volts.

5) 0.3 plus 6.6 equals 6.9 volts, which is noticeably larger than your 5 volt supply. This explains why your transistor is in saturation.

6) So replace Re with. let's say, a 330 ohm resistor. Then the current will be about 1 mA, and the collector resistor should drop about 2 volts. You should wind up with about 2 to 2.5 volts across the transistor, and have a signal gain in the range of about 7.